Corrosion Susceptibility in High Temperature Liquid/Vapor Environments of Materials for Tubing of Heavy-Water Collection System in CANDU Nuclear Power Plant

Corrosion especially pitting on the inner surface was regarded as the first step of the failure process of 316L and 304L stainless steel tubes of heavy-water collection systems in CANDU reactors of a power plant. In this work, four materials including the 316L, 304L, carbon steel A106B and Ni-based Alloy 690 were tested in 14 designed liquid/vapor LiOH-containing environments at both 250 and 150°C, to obtain basic information on the corrosion susceptibilities as a function of temperature, media state, Cl⁻ and Li⁺ contents, for materials selection. Results showed that the corrosion susceptibility rank were A106B at the top, 690 at the bottom and the stainless steels between them. The corrosion susceptibility was lower in a liquid solution than in the vapor above the liquid, and was relatively lower at 150°C than at 250°C for the same media. Chloride promoted corrosion significantly but LiOH showed some protect effect. In the Cl-free solutions at 150°C, A106B exhibited almost no corrosion in the liquid but localized corrosion in the vapor above. It corroded apparently when adding 3%NaCl into the solution or raising temperature to 250°C. The stainless steels showed no corrosion in both the liquid and vapor states of Cl-free solutions at both 150 and 250°C, but exhibiting pitting in the vapor when 3%NaCl added to the solution. 690 always exhibited excellent corrosion resistance during long term tests in various environments.